Radioisotopic heat source and method of production



Jan. 7, 1969 J. J. FITZGERALD ET AL 3,421,001

RADIOISOTOPIC HEAT SOURCE AND METHOD OF PRODUCTION Filed March 16, 1964Sheet of 4 FIGI FIG. 2

FIGB

Tm- 17o (127d) 22% EC m5 78% 0.884 Mev a) 0.968 Mev( 3) 0.084 Mev ('Y)Yb-I7O FIG. 4

ATTO R N EYS Jan. 7, 1969 J. .a. FXTZGERALD ET AL 3,421,001

RADIOISOTCPIC HEAT SOURCE AND METHOD OF PRODUCTION Filed March 16, 1964Sheet FIG. 5

rbmzmo KMBOQ IRADIF7TION TIME (DA S) FIG.6

irom EYS Jan. 7, 1969 J. J. FITZGERALD ET AL 3,421,001

RADIOISOTOPIC HEAT SOURCE AND METHOD OF PRODUCTION Filed March 16, 1964Sheet 3 of 4 Tm-I'Tl (L9 years) 0.066? Mevky) FIG? J 1969 J. J.FITZGERALD E AL RADIOISOTOPIC HEAT SOURCE AND METHOD OF PRODUCTION FiledMarch 16, 1964 POWER DENSITY POWER DENSITY 0F Tm-l69 VS.

NOT CORRECTED FOR FLUX DEPRESSION.

TIME PARAMETRIC IN NEUTRON FLUX.

l l IIIIIIi I Sheet 4 of 4 0 m 0 2 N a 8 8 E LO Hf z E 0 1 0 5 IO 4 n: E

O o m gmmhwmqm GSI- L QIULI- L) SllVM NI ALISNBCI HBMO INVENTORS UnitedStates Patent 3,421,001 RADIOISOTOPIC HEAT SOURCE AND METHOD OFPRODUCTION Joseph J. Fitzgerald, Winchester, and Gordon L.

Brownell, Weston, Mass, assignors to Iso/Serve,

Inc., Cambridge, Mass, a corporation of Massachusetts Filed Mar. 16,1964, Ser. No. 352,725

US. Cl. 250106 18 Claims Int. Cl. G21h 5/00 The present inventionrelates to a means and method of providing heat and ionizing radiationsources for energy generation and material irradiations and moreparticularly relates to a means and method of providing coldencapsulated radioactive heat sources which may be used as energygenerating components; as product and food sterilizers; and as plasticand detergent irradiators.

There has been an increasing demand for radioactive heat sources forenergy purposes which are included in such devices as thermionic andthermoelectric generators. Heretofore, attempts have been made toprovide such heat sources by the fabrication of radioisotope capsules orwafers that are incorporated into such devices as energy sources. Greatdifficulty, however, has been encountered in the suitable manufacture ofsuch components in view of their preexisting radioactive nature. Priorto the development of the present concept of cold encapsulation, sourcesof Sr-90, Ce-144, and Pu-238 that are radioactive were radioactively hotencapsulated. Because of the inherent dangers in the handling andencapsulation of these radioactive components, elaborate facilities arenecessary at substantial cost to provide adequate radiation protection.These elaborate facilities greatly reduce the flexibility in design andfabrication of such heat sources and, consequently, result in greaterproduction and handling costs.

There has also been an increasing demand for radiation sources to reducethe bacteria content in food and some nonedible products and to changethe properties of certain materials such as plastics and detergents.Heretofore, costly high energy isotope sources that require considerableshielding have been used for such purposes. In some instances,accelerators have also been used for the same purposes. But :both highenergy isotopes and accelerators require considerable space andshielding. However, the use of cold encapsulated relatively low energysources permit the relatively inexpensive preparation of a source thatneeds a relatively small amount of shielding. Space requirements andshielding weight is so low that the unit can be readily moved from placeto place.

It is, therefore, an object of the present invention to provide a meansand method of fabricating wafers or discs that can be made radioactivefor use as heat sources for energy generation or for other applicationsrequiring ionizing radiation sources.

It is also an object of the present invention to provide a means andmethod for fabricating heat and ionizing radiation sources that reducehazards normally associated with preparing radioisotope sources. Thepresent invention also eliminates the necessity for elaborate nuclear orradiation facilities with a consequent reduction in cost for thepreparation of radioactive sources.

A further object of the present invention is to provide a method ofencapsulating a stable source or non-radioactive material in anencapsulating material that is also stable or nonradioactive. Anotherobject of this invention is to provide for the cold encapsulation of astable material having a relatively high thermal neutron cross sectionin a stable encapsulating material having a relatively low thermalneutron cross section with a relatively short half life. With the coldencapsulation method, the radio- 3,421,001 Patented Jan. 7, 1969actively inert or stable material may be subjected thereafter to aneutron flux of a specific level to induce sufficient activity into thematerial for its use as a heat source or a materials irradiator.

The present invention also provides a means and method by which stablematerial suitable for exposure to neutron flux for use as a radiationsource may be fabricated and stored prior to exposure to neutron fluxfor indefinite periods of time.

In the present invention there is provided a means and method in which astable material which has a relatively high thermal neutron crosssection and a relatively long half life may be encapsulated prior toneutron exposure in such a manner that the encapsulation containing thestable material is essentially inactive and safe for handling. Thisstable material may, after fabrication and encapsulation, be subjectedto a neutron flux. As this means and method does not containradioactively hot material but rather inert or cold material, it iscalled a cold encapsulation process.

These and other objects of the present invention will be more clearlyunderstood when considered in conjunction with the accompanying drawingsin which:

FIG. 1 is a cross section of a stable compressed and sintered compoundprior to irradiation;

FIG. 2 is a cross section of an encapsulation formed in accordance withthe present invention;

FIG. 3 is a schematic illustration of a plurality of encapsulationsduring irradiation;

FIG. 4 is a graphical illustration of a decay scheme for thulium-170;

FIG. 5 is a graphic representation of power density of thulium oxideversus time, parametric in neutron flux;

FIG. 6 schematically illustrates a group of packaged capsules;

FIG. 7 is a graphical illustration of a decay scheme for "Fm-171; and

FIG. 8 is a graphic representation of power density of thulium oxideversus time, parametric in neutron flux.

This invention is directed primarily toward an encapsulation of acompound or isotope which is stable, compressed and sintered and whichis made adaptable for irradiation into an active heat source in itsencapsulated form. As the source is fabricated and encapsulated beforethe isotope is subject to irradiation it may be referred to as a coldencapsulation process.

The means and methods described may be used in connection with severaldilferent materials, although certain isotopes have properties whichmake their use particularly desirable as practical heat sources. Thematerial should have a thermal neutron cross section which is high,preferably in excess of 5 barns and a half life preferably in excess ofdays. The material also should have no significant gaseous daughterproducts formed during emission. For certain aplications, notably foruse in thermionic generators, the material should have a high meltingpoint, preferably in excess of 1700 0, although for other applicationsthis is not a consideration. It has been found that thulium-169 preparedas thulium oxide Tm O is the most practical and technically feasiblematerial for use in cold encapsulation. This is due in part because itis the only stable isotope of thulium (100% abundant) and has a thermalneutron cross section of 118 barns which assures a sufiicient activationto thulium-170, or with neutron fiuxes in the range of 2 to 5 10 n./cm./sec., suitable and desirable quantities of thulium-171 can be inducedinto the thulium oxide encapsulated wafer. Thulium-169 in the form ofthulium oxide particles is prepared by any suitable commercial method.Thulium oxide is used because pure thulium-169 as a solid metal mayreact with the material forming the casing of the encapsulation andwould fuse to the encasing material.

The thulium oxide is compressed and sintered under heat and pressureconditions into a wafer or disc form as shown in FIG. 1. The specificdimensions are determined at least in part by the particular ultimatepower or radiation level for which the unit is designed. Generally, fluxdepression during irradiation may be lessened by making the waferrelatively thinner, while still maintaining the structural integrity ofthe unit for maximum efficiency. A typical wafer would, for example,have a thickness of 2 to mm. and a diameter of 1" to 2 /2". The closerthe thulium oxide approaches its theoretical density, the better will bethe power output efficiency since the maximum power per unit dimensionis a function of density. In order to achieve maximum power density, itis desirable to compress the thulium oxide to a density of at least 80%of its theoretical maximum density, and most preferably to a range of90% to 95% of theoretical density. The thulium oxide wafer is formed bycompressing thulium oxide powder, utilizing conventional equipment at anelevated temperature of just below the melting point of thulium oxidewhich is in the range of approximately 2300 C. to 2600 C. Sintering thecompressed wafer may be conducted in air, vacuum or in an inertatmosphere.

The thulium oxide wafer 2 in FIG. 1 is placed in and secured to a casing3 of FIG. 2. The material of which the casing 3 is formed must have ahigh melting point which is at least in excess of the melting point ofthe material contained within it. In the case of the preferredembodiment, the melting point must be in excess of that of thuliumoxide. The casing material should not be conducive to significantactivation and, therefore, it should have a very low cross section forabsorption of thermal neutrons and a short half life. Preferably, thecasing should be of material having a thermal neutron cross section ofless than .2 barn and a half life of less than 3 days. The casingmaterial must not react with the fuel material or isotope which formsthe wafer and also must be capable of being joined to form a sealedcasing with the fuel material contained within it. It has been foundthat molybdenum is the most preferred material for the casing althoughother materials such as zirconium and tungsten might also be used.Molybdenum is preferred primarily because of its high melting point,relatively short half life and other suitable characteristics.

The casing 3 should have relatively thin walls, with the walls having athickness in the range of 5 millimeters or perhaps less. By making thecasing as thin as possible, without weakening its structural integrity,flux depression during irradiation may be minimized and, therefore,neutron absorption of the fuel material may be maximized and the powerdensity increased. The sidewalls 4 and bottom 5 may be integrally formedand may be joined to the cover 6, with all portions of the casing 3joined to the fuel material 2 by conventional methods. Conventionalmeans may, for example, comprise electron beam welding in a vacuum. Itis most desirable to join the casing to the fuel material, in this casethulium oxide, to assure an appropriate contact between the fuel waferand the container, so as to conduct heat effectively from the fuelmaterial to the container for greater efficiency.

The encapsulation 7 illustrated in FIG. 2, when initially formed is notradioactive, and, therefore, may be termed a cold encapsulation. Thiswafer requires no shielding and may be handled as an inactive material.

Inactivated capsules 7 may be placed in a reactor 8, as schematicallyillustrated in FIG. 3, for activation and held in the reactor until justprior to use. For most applications, the capsules are placed in areactor for a period of not less than 35 days and not more than 150 daysand are exposed to a flux equal to or greater than 10 n./cm. /sec. forproduction of thulium-170. For production of thulium-171 the capsulesare exposed to a flux in the order of 10 n./cm. /sec. for a period of 30to 90 days. In order to minimize flux depression a spacing of in theorder of at least 5 to 1 between capsules in the reactor is utilized.Thus, in the preferred embodiment, the capsules 7, having a thickness ofapproximately 2 millimeters are spaced a distance apart of approximatelyeight millimeters. The eight millimeters space 9 between wafers shouldbe occupied by a material which is capable of moderating the neutronsand cooling the areas, so that the neutrons will be slowed down to aspeed at which they will more effectively be absorbed by the fuelmaterial. Any suitable cage may be used to separate and support thecapsules in the reactor, including for example, a cage of molybdenum oraluminum. The moderating material between the wafers should be ahydrogenous material, such as water. In the preferred embodiment, theflux depression is expected to be in the order of 50%. In actuallydetermining the flux depression, not only must the spacing of the waferswithin the reactor be considered, but also the thickness of the wafer isa controlling function.

FIG. 5 illustrated the power density of thulium oxide versus time,parametric in neutron flux that may be attained. This graph is basedupon the assumption that the flux exists at the target material. As isevident from an examination of FIG. 5, thulium oxide has a reasonablyhigh specific activity that can be achieved at reactor fluxes which aregenerally available today with the radiation times within reason.

When exposed to the neutron flux, the thulium-169 in the form of thuliumoxide absorbs neutrons which convert the stable thulium oxide to activethulium-170. The reaction that takes place by irradiating thulium-169 ina new tron flux to produce thulium-170 is:

The thulium-170 decays to a stable daughter ytterbium- 170 asillustrated in FIG. 4. However, thulium-170 has a cross section of barnswhich indicates some of the thulium-169 is converted into thulium-171,according to the equation:

The Tm-170 and Tm-l71 which are formed by subjecting the wafer to aneutron flux appear to be the most practical and technically feasiblematerial for use as a heat source, particularly in connection withthermionic generators. This is due at least in part to the attainablepower densities which may be achieved with thulium-170 fuel wafers, andthe longer half life of Tut-171 and the radiation safety aspects.

The activated encapsulated wafers 7 may be stacked and contained withinan outer casing as illustrated in FIG. 6. The encapsulated wafers 2 ofthe preferred embodiment which may have a width of 2 millimeters arestacked to various heights.

The outer casing 15 within which the wafers 2 are contained shouldpreferably be formed of the same material as the casing material 3. Inaddition, the thickness of this outer casing should be substantially thesame thickness as the thickness of the casing 3 and may be sealed in asimilar way to casing 3.

While thulium-169, the only stable isotope of thulium, is a preferredstable material for use in the present invent-ion, other materials havealso been considered. Thulium-17l can also be produced by the use ofstable erbium or enriched Er-170 in the following manner:

The above process requires a chemical separation of Tin-171 from theerbium isotopes or Er-l70. Consequently, the encapsulation is ultimatelyaccomplished with radioactively hot material, Tm-l7l. Since the energyemitted per disintegration from Tm-171 is very low, the radiation safetyproblems are very small.

What is claimed is:

1. A method of preparing a radioactive heat source encapsulationcomprising:

preparing a wafer of stable thulium oxide for encapsulation bycompressing and sintering particles of thulium oxide under the influenceof heat,

placing said compressed and sintered wafer in a capsule of molybdenum,joining said wafer to the walls of said capsule,

sealing said capsule, and, thereafter subjecting said nonradioactivethulium oxide to a neutron flux of at least substantially n./cm. /sec.for a period of between approximately 35 and 150 days.

2. A method as set forth in claim 1 wherein said nonradioactive thuliumoxide is subjected to a neutron flux of at least 10 n./cm. /sec. for aperiod of between 30 and 90 days for production of thulium-171.

3. A method of preparing a radioactive heat source encapsulationcomprising:

preparing a Wafer of stable thulium oxide for encapsulation bycompressing and sintering particles of said thulium oxide into anintegral wafer,

placing said wafer in a capsule formed of material which has a meltingpoint in excess of 2300 C., is not chemically reactive with said thuliumoxide, is capable of being joined to said thulium oxide, has a crosssection of less than .2 barn to thermal neutrons and half life of lessthan 3 days, sealing said capsule, and thereafter subjecting saidnonradioactive thulium oxide to a neutron flux of at least substantially10 n./cm. /sec. for a period of between approximately 35 and 150 days.

4. A method as set forth in claim 3 wherein the specific activity ofsaid thulium oxide after radiation is in the order of 5 to 15 watts/cc.from Tm-170.

5. A method as set forth in claim 3 wherein said thulium oxide issubjected to a neutron flux of at least 1O n./ cm. sec. for a period ofat least 30 days for production of thulium-171.

6. A method as set forth in claim 5 wherein said thulium oxide issubjected to said neutron flux until the specific activity of saidthulium oxide after radiation is in the order of .5 to 1.5 watts/cc.from thulium-171.

7. A method as set forth in claim 4 wherein said capsule is formed ofmaterial selected from a group consisting of molybdeum, zirconium andtungsten.

8. A method as set forth in claim 7 wherein said thulium oxide iscompressed to a density in excess of 80% of theoretical maximum density.

9. A method of preparing a radioactive heat source encapsulationcomprising:

preparing a wafer of stable material having a cross section of greaterthan 5 barns to thermal neutrons, placing said Wafer in a capsule formedof material which is nonreactive with said water material, is capable ofbeing sealed around said wafer material,

has a cross section of less than .2 barn to thermal neutrons and a halflife of less than 3 days,

sealing said capsule, and

thereafter subjecting said nonradioactive material to a sufiicientneutron flux for a period whereby the specific activity of said materialafter radiation is in excess of 0.5 watt per cc. of said material.

10. A nonradioactive encapsulation heat source adapted to be convertedby exposure to neutron flux to an isotope fueled heat source,comprising:

a wafer of a stable compressed sintered isotope having a cross sectionof greater than 5 barns to thermal neutrons and a half life in excess of100 days,

said water entirely contained and enclosed in a sealed capsule ofmaterial which has a melting point in excess of 2300 C., is nonreactivewith said compound, is capable of being joined to said isotope, has across section of less than .2 barn to thermal neutrons, and a half lifeof less than 3 days.

11. An encapsulation as set forth in claim 10 wherein said wafer isjoined to the inner Walls of said capsule.

12. An encapsulation as set forth in claim 11 wherein said wafer isformed of thulium oxide.

13. An encapsulation as set forth in claim 12 wherein said wafer isformed of thulium oxide and said capsule material is selected from agroup consisting of molybdenum, zirconium and tungsten.

14. An encapsulation as set forth in claim 13 wherein a plurality ofwafers are contained within said capsule.

15. An encapsulation as set forth in claim 10 wherein said waferconsists essentially of said sintered isotope.

16. A method in accordance with the method of claim 9 wherein saidstable material consists essentially of at least one isotope.

17. A method in accordance with the method of claim 3 wherein said waferconsists essentially of thulium oxide.

18. A method in accordance with the method of claim 1 wherein said waferconsists essentially of thulium oxide.

References Cited RALPH G. NILSON, Primary Examiner.

A. B. CROFT, Assistant Examiner.

US. Cl. X.R.

10. A NONRADIOACTIVE ENCAPSULATION HDEAT SOURCE ADAPTED TO BE CONVERTEDBY XPOSURE TO NEUTRON FLUX TO AN ISOTOPE FUELED HEAT SOURCE, COMPRISING:A WAFER OF A STABLE COMPRESSED SINTERED ISOTOPE HAVING A CROSS SECTIONOF GREATER THAN 5 BARNS TO THERMAL NEUTRONS AND A HALF LIFE IN EXCESS OF100 DAYS, SAID WAFER ENTIRELY CONTAINED AND ENCLOSED IN A SEALED CAPSULEOF MATERIAL WHICH HAS A MELTING POINT IN EXCESS OF 2300*C., ISNONREACTIVE WITH SAID COMPOUND, IS CAPABLE OF BEING JOINED TO SAIDISOTOPE, HAS A CROSS SECTION OF LESS THAN .2 BARN TO THERMAL NEUTRONS,AND A HALF LIFE OF LESS THAN 3 DAYS.